Once a hair has grown, its color never changes. The color is added deep in the root while the hair grows.
Where is the color made?
Tiny pigment cells there make a color called melanin. They push it into the hair as it grows.
Why does the color stop?
Each time a new hair starts growing, a few spare cells called stem cells turn into new pigment cells. As we get older, more and more of those stem cells stop working. So new hairs grow with little or no melanin, and they come out gray or white.
Can stress make hair gray?
In mice, strong stress turned all those stem cells into pigment cells at once. So none were left for the next hairs, and those hairs grew in gray. This was a study in mice, and it does not mean that a little stress will turn your hair gray.
Sources
- American Academy of Dermatology: What causes gray hair, and can I stop it?
- University of Tokyo: Why does our hair turn gray and thin out as we age?
- NYU Langone: Study Links 'Stuck' Stem Cells to Hair Turning Gray
- Harvard Gazette: New findings suggest how stress may trigger gray hair
- Nature: Hyperactivation of sympathetic nerves drives depletion of melanocyte stem cells
- Flushing Hospital: Myth or fact: Can hair turn white from fright overnight?
- Healthline: Marie Antoinette Syndrome
- ScienceAlert: Evidence in Humans Shows Stress Really Can Turn Hair Gray
- Science News: How stress turns hair gray
- The Scientist: How Does Stress Cause Gray Hair?
